OpenGL (Open Graphics Library) is a standard specification defining a cross-language cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL was developed by Silicon Graphics Inc. (SGI) in 1992 and is widely used in CAD, virtual reality, scientific visualization, information visualization, and flight simulation. It is also used in video games, where it competes with Direct3D on Microsoft Windows platforms (see Direct3D vs. OpenGL).
>There are other open-standards and open-source alternatives to DirectX, such as SDL
Actually, SDL is just a wrapper library designed to abstract the details of graphics, sound, input, etc. Internally it will use DirectX or OpenGL to draw graphics, for example.